The measurement and monitoring of various properties of animal tissue has become of high importance in many fields. Therefore, there is an increasing desire to provide new or improved methods of measuring characteristics of animal (including human) tissue. In particular, there is a desire to provide methods which allow an easy and practical application with the minimum of inconvenience to the test subject. A particular measurement that is of interest is the measurement of a propagation velocity for a surface wave in animal tissue. Such measurements may provide important indications of various characteristics and may for example assist in the diagnosis or early detection of various health risks.
For example, it is known that high blood pressure is a common risk factor for heart attacks, strokes and aneurysms, and therefore diagnosis and monitoring of this is critically important. Many cardio-vascular diseases originate from a stiffening of the arterial walls, which in turn is related to the blood pulse wave velocity (PWV) via the Moens-Korteweg equation:
      PWV    =                  Eh                  ρ          ⁢                                          ⁢          d                      ,where E denotes Young's elasticity modulus of the vessel, h the wall thickness, p the blood density and d the vessel diameter.
A number of methods have therefore been developed to measure blood pulse wave velocity. Due to the relation with the vessel's elastic properties, both invasive and non-invasive methods have been developed. Generally, these involve measuring the passing pressure wave at multiple positions and extracting the velocity of the pulse from the ratio of the displacement and the time delay observed in the recordings. It has been proposed to measure the pressure wave using invasive catheters, mechanical tonometers, Ultrasound Doppler analysis (as disclosed in Xu, M., 2002, “Local measurement of the pulse wave velocity using ultrasound Doppler”, Ph.D dissertation, Massachusetts Institute of Technology.), or (piezo-electric) pulse detection devices applied non-invasively to the skin (as disclosed in McLaughlin, J., McNeill, Braun, B and McCormack, P.D, 2003, “Piezoelectric sensor determination of arterial pulse wave velocity”, Physiol. Meas. 24, 693-702).
However, the proposed approaches tend to not be optimal. For example, they tend to be inconvenient to the test object (e.g. requiring invasive operations), cumbersome to perform, to provide results that are not as accurate or reliable as would be preferred and/or to require complex and/or costly equipment. In particular, most methods require different sensors to be carefully synchronized to allow the detection of the propagation velocity. Such synchronization tends to be complicated and costly to achieve.
Hence, an improved approach for determining a propagation velocity for a surface wave in animal tissue, and specifically for determining a propagation velocity for a pulse wave, would be advantageous, and in particular a system allowing increased flexibility, reduced resource demand, reduced cost, facilitated implementation, reduced complexity, reduced inconvenience to a test subject, relaxed camera requirements and/or improved performance would be advantageous.